I need to spec out some high pressure ball valves for dry CO2 injection (between 1000 and 3000 psig) and was wondering about trim.
No prior experience with high pressure CO2 per se but after researching, I'm thinking electroless nickel on carbon steel for the ball valve and stem (since the service is relatively benign) with nylon seats but then I'm kind of stuck on the body seals. The typical Buna-N and Viton materials aren't compatible with CO2 (rapid depressurization issues) and beyond that it isn't clear from the manufacturer's literature what alternatives might be commonly available. Everyone seems to have a "proprietary" reference in their catalogues.
I guess my alternative would be to leave it open ended with a requirement that the elastomer be qualified to some acceptable test standard (NORSOK M-701, Shell, John Walker, etc.).
For larger sizes one would probably tend to search for 'cheaper' material and sealing combinations than for lower sizes.
If you are discussing lower sizes the answer would probably be more self-evident: straight stainless steel or better qualities with PTFE, which would be adequate also with impurities, water added or acids formed. Why should it be very different for larger sizes, except for cost/lifetime reasons?
As a preliminary resarch for market availabillity I would have asked for a sample price or budget price for typical size and quantity (for further qualification before final bid) from two or several local suppliers, aiming for higher quality (25 year lifetime expected)  with option to also offer for cheaper quality /lower lifetime expectations.

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